Epicyclic gearboxes are well known and are frequently used for their compact design and efficient high transmission ratio capabilities which is particularly useful in the environment of gas turbine engines. Epicyclic gear trains are also advantageous due to their versatility. Planetary and star gear trains are alternate arrangements used in epicyclic gearboxes. Both generally comprise three gear train elements: a central sun gear, an outer ring gear, and a plurality of planet gears supported by a planet carrier between and in meshing engagement with both the sun gear and the ring gear. A rotary input can be connected to any one of the three elements. Holding one of the remaining two elements stationary with respect to the other two, permits the third to serve as an output. In planetary gear trains, the central sun gear provides the input, the outer ring gear is held stationary, and the planet gears that rotate therewithin cause their planet carrier to rotate, which provides the reduced speed rotary output. In star gear trains, the sun gear provides the input. However, the planetary carrier is held stationary, and the outer ring gear provides the rotary output in a direction opposite that of the input sun gear.
While conventional epicyclic gear trains are significantly compact, particularly in relation to their high gear reduction capabilities, in relation to other types of gear trains, potential improvements remain possible in order improve the compactness of such gear train arrangements.
Further, certain shortcomings exist with known epicyclic drive trains. For example, as with many mechanical elements that transfer torque, a small but nevertheless significant amount of torsional deflection commonly occurs due to the elasticity of the material of the carrier, as a result of twist between upstream and downstream plates of the gear carrier, when the gear train is under load. The gear carrier generally twists around its central axis, causing the individual axis of rotation of the gears to lose parallelism with the central axis of the gear carrier. This torsional deflection results in misalignment at gear train bearings and at the gear teeth mesh, which leads to efficiency losses and reduced life of the parts.
Attempts to address this problem of planetary carrier torsional deflection are known. U.S. Pat. No. 5,466,198 issued Nov. 14, 1995 to McKibbin et al, for example, clearly sets out the problem and proposes a planetary gear train drive system which isolates the planetary carrier from torsional deflections. A torque frame or torque transfer structure is connected to a rotating load, such as a bladed propulsor. Pivotal joints, circumferentially disposed with respect to the carrier, each pivotable about a radial axis, connect axially extending arms of a torque frame to the planetary carrier. The pivotal joints permit the planetary carrier to be isolated from torsional deflections. However, further reductions in the torsional deflections resulting in the planetary carrier are possible.
There remains a need for a more compact epicyclic gear train arrangement that is capable of transferring torque while further reducing torsional deflections therewithin.